Pulmonary alveoli are the smallest structural unit of the lungs and belong to the respiratory segment of the same. This means that the alveoli serve to exchange gases between inhaled air and blood. Each lung contains around 300-400 million alveoli.


The lungs can generally be divided into two large lobes, the left and right lobes. In these lobes, the bronchial tree branches out into ever smaller sections. The bronchial tree is the entirety of the air-conducting system. It begins after the trachea with the two main bronchi. Then each main bronchus splits into two (left side) or three (right side) lobe bronchi. These correspond in number to the lung lobes. The lobular bronchi further divide into the segmental bronchi and the lobular bronchi. This is followed by the bronchioli. While the Terminal bronchioles are still part of the air-carrying part of the bronchial tree Bronchioli respiratoriithat come after that are already part of the respiratory section of the bronchial tree.

The difference between air and respiratory sections is that the Wall construction the respiratory Sections allows a gas exchange between blood and breathing air, during the air conductionAs the name suggests, it can only direct the air you breathe to the alveoli and back again.

On the walls of the Bronchioli respiratorii individual alveoli are already attached and serve this purpose Gas exchange.

Then go from the bronchioli respiratorii Alveolar ducts (Alveolar duct) and those attached to it Alveolar sac (Alveolar sacculi) from. These only consist of many individual ones Alveoli (Alveoli) and are also used for gas exchange.

All the alveoli that become one Terminal bronchioli belong, become one Acinar summarized as a unit. This corresponds to the smallest lung unit.

The alveoli have a diameter of up to, depending on the filling 250 µm.

The alveoli are surrounded by a fine one Capillary network. This capillary network is made by the pulmonary arteries (Pulmonary arteries) fed. These do that deoxygenated blood from the heart to the lungs to be enriched with oxygen. After the capillary network, the Pulmonary veins (Pulmonary veins) the now oxygen-rich Blood back to the heart.

Histology (detailed structure)

A Alveoli is a honeycomb-like protuberance of the bronchial system. The alveoli have a very thin wall. This thin wall is one for optimal conditions rapid gas exchange necessary between blood and breath.

The wall of the alveoli is formed by different cells. With 90% the majority do the Type I pneumocytes out. These large and thin cells are that Endothelium very similar and line the alveoli. These Type I pneumocytes are no longer divisible. They are responsible for the gas exchange and belong to the Blood-air barrier.

About 7% of the cells are Type II pneumocytes. These cells are compared to Type I pneumocytes hhigher and not so flat. Type II pneumocytes are for the formation of Surfactant responsible. Surfactant is a surface active substance that is made up of Phospholipids and Surfactant proteins consists. This substance also lines the alveoli and reduces the surface tension of the lungs. It thus ensures that the alveoli do not collapse, that is, would collapse.

Type II pneumocytes are also divisible and can be lost due to defect coverage Type I pneumocytes replace.

Can also be used as additional cells in the alveoli Alveolar macrophages occurrence. These cells belong to the immune system, that is, to the defense system of the lungs. Alveolar macrophages can phagocytose pathogens that have penetrated the alveoli and thus keep the lungs and alveoli clean.

The alveoli are separated from one another by these walls. However, in these walls there are small pores, so-called Kohn“Pores through which the alveoli are in contact with each other.


The Alveoli serve to exchange gas between the air that is inhaled and passed through the bronchial system and the blood in the capillaries.

The gas exchange takes place via the membrane that separates the alveoli from the capillaries. This is the so-called Blood-air barrier, i.e. the path that the oxygen has to cover in order to pass from the air we breathe into the blood. The blood-air barrier consists of the following parts: the Cell processes of type I pneumocytes, a thin one Basal lamina and the Cell processes of the endothelial cells. The endothelial cells are part of the wall structure of the capillaries. This blood-air barrier is only 0.2 to 0.6 µm thick. Because of this short path that the gas has to cover and the density of the capillary network around the Alveoli around, becomes a faster and efficient gas exchange guaranteed. This is because the time in which the blood in the capillaries is available for gas exchange is only very short and is around 0.75 seconds.

The gas exchange means that oxygen in the inhaled air comes through the bronchial system into the alveoli. Here the gaseous oxygen molecules can pass through the Blood-air barrier pass into the blood. In return, the blood becomes Carbon dioxide released, which is brought out of the body in the exhaled air. For an efficient gas exchange is a good one Perfusion and ventilation necessary. Perfusion means that Alveoli be supplied with sufficient blood through the capillary, i.e. enough blood to the Alveoli flowing along. Ventilation means that the lungs and thus the alveoli are adequately ventilated, i.e. sufficient air flows into and out of the lungs.


Alveoli make up the smallest unit of the lungs. They are formed by different cells and are responsible for the gas exchange between the air we breathe and the circulating blood. This requires functional alveoli and a blood-air barrier that is as thin as possible, as well as an adequate supply of blood (Perfusion) and oxygen (ventilation) necessary.